A mobile station, such as a cell phone or a personal digital assistant (PDA), may be used to communicate with a cellular wireless network. The mobile station typically communicates with the network over a radio-frequency (RF) air interface according to a wireless communication protocol such as Code Division Multiple Access (CDMA), perhaps in conformance with one or more industry specifications such as IS-95 and IS-2000. A wireless network that operates according to these specifications is also referred to as “1xRTT (1x) network,” which stands for “Single Carrier Radio Transmission Technology”. Such a network (referred to herein as a “CDMA network”) typically provides communication services such as voice, Short Message Service (SMS) messaging, and packet-data communication.
A typical CDMA network includes a plurality of base stations, each providing one or more wireless coverage areas—for example, one or more sectors. When a mobile station is positioned in a sector, it can communicate over the RF air interface with the base station, and in turn over one or more circuit-switched and/or packet-switched signaling and/or transport networks to which the base station provides access. The base station and the mobile station conduct these communications over a frequency known as a carrier. A base station may provide service in a sector on one carrier, or on more than one carrier. A given instance of a given carrier in a given sector may be referred to herein as a “sector/carrier.”
Communication, including voice and packet-data communication, between the mobile station and the base station is separated into forward-link communication (from the base station to the mobile station) and reverse-link communication (from the mobile station to the base station). Each carrier over which this communication takes place is typically a pair of distinct frequencies—one for the forward link and the other for the reverse link. This approach is known as frequency division duplex (FDD).
In a typical CDMA network, using a configuration known as radio configuration 3 (RC3), a base station can, on each sector/carrier, transmit forward-link data on a maximum of 64 distinct channels at any given time. Each of these 64 channels corresponds to a unique 64-bit code known as a Walsh code. Of these, typically, 61 channels are available for use as traffic channels (to carry user data). The other 3 channels—the pilot channel, the paging channel, and the sync channel—are used for administrative functions and are generically referred to as overhead channels. The transmitting power of a base station on a given sector/carrier is divided among the pilot, paging, and sync channels, and among the mobile stations to which the base station is transmitting voice data and/or packet data on traffic channels.
In general, the pilot channel functions to alert mobile stations in a given sector of the presence of a service-providing base station. Typically, the pilot channel conveys a value known as a pseudorandom number (PN) offset, which identifies the sector; in particular, by being offset from CDMA system time by a certain amount of time, the pilot channel conveys the PN offset. Mobile stations generally use the pilot channel to coherently detect and demodulate the signal on a given sector/carrier. The paging channel is typically used to transmit overhead messaging, such as incoming-call and message-waiting pages, as well as SMS messages, to mobile stations. The paging channel typically also broadcasts values known as the system identification code (SID) and the network identification code (NID), which, taken together, help identify a given sector. The sync channel generally provides information that enables the base station and mobile stations to be precisely synchronized on CDMA system time. A mobile station “acquires” a cellular wireless network by, among other things, detecting the pilot channel of the sector/carrier and by reading the synchronization information from the sync channel and the overhead messaging information from the paging channel.
When a base station instructs a mobile station to use a particular traffic channel for a particular communication session, the base station does so by instructing the mobile station to tune to a particular one of those 64-bit Walsh-coded traffic channels. It is over that assigned traffic channel that the base station will transmit forward-link data to the mobile station during the ensuing communication session. Note that, in addition to including the forward-link channel, the traffic channel also includes a corresponding Walsh-coded reverse-link channel, over which the mobile station transmits data to the base station. These traffic channels may be used for different types of communication, among which are second-generation (2G) voice, 2G data, third-generation (3G) voice, and 3G data.